1. Field of the Invention
Embodiments of the present invention generally relate to the field of seismic data processing, and more specifically to prediction and removal of multiples in seismic reflection surveying.
2. Description of the Related Art
Seismic surveying is a method for determining the structure of subterranean formations in the earth. Seismic surveying typically utilizes seismic energy sources which generate seismic waves and seismic receivers which are strategically positioned to detect the seismic waves. The seismic waves propagate into the formations in the earth, where a portion of the waves reflects from interfaces between subterranean formations. The amplitude and polarity of the reflected waves are determined by the differences in acoustic impedance between the rock layers comprising the subterranean formations. The acoustic impedance of a rock layer is the product of the acoustic propagation velocity within the layer and the density of the layer. The reflected seismic waves are detected by the seismic receivers, which convert the reflected waves into representative electrical signals. The signals are typically transmitted by electrical, optical, radio or other means to devices which record the signals. Through analysis of the recorded signals, the shape, position and composition of subterranean formations can be determined.
Land seismic surveying is a method for determining the structure of subterranean formations beneath the surface of the earth. Typically, the seismic energy source used in land seismic surveying is an apparatus capable of delivering a series of impacts or mechanical vibrations to the surface of the earth or the detonation of an explosive charge near the surface of the earth, while the seismic receiver used is a motion sensor, such as a geophone or an accelerometer. The seismic sources and seismic receivers are typically placed on the surface of the earth, although either source or receiver may be placed in a borehole for vertical seismic profiling. Both the seismic sources and the seismic receivers are typically repositioned to cover the survey area.
Marine seismic surveying is a method for determining the structure of subterranean formations underlying bodies of water. Marine seismic surveying typically utilizes seismic energy sources and seismic receivers located in the water and are typically towed behind a vessel or positioned on the water bottom from a vessel. The energy source is typically an explosive device or compressed air system that generates seismic energy, which are configured to propagate as seismic waves through the body of water and into the earth formations below the bottom of the water. As the seismic waves strike interfaces between subterranean formations, a portion of the seismic waves reflects back through the earth and water to the seismic receivers, to be detected, transmitted, and recorded. The seismic receivers typically used in marine seismic surveying are pressure sensors, such as hydrophones. Both the sources and receivers may be repositioned to cover the survey area.
Seismic waves, however, do not reflect only from the interfaces between subterranean formations, as would be desired. Seismic waves also reflect from the water bottom and the water surface, and the resulting reflected waves themselves continue to reflect. Waves that reflect multiple times are called xe2x80x9cmultiplesxe2x80x9d. Waves that reflect multiple times in the water layer between the water surface above and the water bottom below are called xe2x80x9cwater bottom multiplesxe2x80x9d or xe2x80x9cwater layer multiples.xe2x80x9d Water layer multiples have long been recognized as a problem in marine seismic processing and interpretation, and consequently, multiple attenuation methods have been developed to handle water layer multiples.
In general, current multiple attenuation methods can be roughly divided into two categories: filtering methods and wave-equation prediction methods. Filtering methods rely on periodicity of the multiples or on significant velocity differences between primary reflections (xe2x80x9cprimariesxe2x80x9d) and multiples. Primary reflections are those seismic waves that have reflected only once, from the water bottom or an interface between subterranean formations, before being detected by a seismic receiver. Predictive deconvolution is a filtering method that assumes that multiples are periodic while primaries are not. This assumption is usually met for data from water depths less than 500 msec (approximately 1,200 feet) and approximately layered subsurface geology. In areas of water depths greater than 500 msec where the velocity difference between primaries and multiples are significant, velocity-filtering methods such tau-p and f-k filtering can be used, where the variable f represents frequency, k represents the wavenumber, p represents the ray parameter, and tau represents the zero offset intercept time.
However, filtering methods generally require determination, or at least an educated guess, of wave propagation velocities in the subsurface media through which the reflected seismic waves pass in their journey from the seismic source to a receiver. These velocities can differ significantly from one type of medium to another. In addition, predictive deconvolution often leads to inadvertent removal of primaries in the process of removing the multiples. Moreover, predictive doconvolution often fails to take into account the nonlinear factor in the reflectivity, which are generally caused by peg-leg multiples.
Wave-equation methods generally use the physical wave-propagation phenomenon to predict and subtract multiples from data. Wave-equation methods can be very accurate, but also very expensive and time-consuming to use compared to filtering methods. Wave-equation methods exploit the fact that primaries and multiples are physically related. Wave-equation methods generally can handle complex geometries and need little or no information about the properties of the subsurface.
Some wave-equation methods, however, require structural information, i.e., information about the subsurface structure, the determination of which is the reason for doing seismic exploration in the first place. Other wave-equation methods require the shape of the source wavelet that will not be a pure delta function because of reverberations and frequency bandwidth limitation. Some wave-equation methods require both structural and source wavelet information. If the shape of the source wavelet is not accounted for, i.e., it is ignored, predicted multiples will not match actual multiples and attempts to remove multiples will fail.
Therefore, a need exists in the art for a method of accurately predicting and removing multiples in a gather of seismic data traces.
Embodiments of the present invention are generally directed to a method for attenuating water layer multiples from a gather of seismic data traces. The method includes predicting a plurality of receiver side water layer multiples in the gather of seismic data traces using a convolutional operator derived from a water layer model, adaptively subtracting the receiver side water layer multiples from the gather of seismic data traces, predicting a plurality of source side water layer multiples using the convolutional operator derived from the water layer model, and adaptively subtracting the receiver side water layer multiples and the source side water layer multiples from the gather of seismic data traces to generate a plurality of primaries in the gather of seismic data traces.
In one embodiment, the method includes forming the gather of seismic data traces in a t-x domain, transforming the gather of seismic data traces from the t-x domain to a tau-p domain, convolving the gather of seismic data traces with a convolutional operator to predict a first set of water layer multiples in the gather of the seismic data traces, adaptively subtracting the first set of water layer multiples from the gather of seismic data traces, removing a water bottom primary from the gather of seismic data traces, convolving the convolutional operator with the gather of seismic data traces after the first set of water layer multiples has been adaptively subtracted from the seismic data traces and after the water bottom primary has been removed from the gather of seismic data traces to predict a second set of water layer multiples in the gather of seismic data traces, adding the first set of water layer multiples to the second set of water layer multiples, transforming the sum of the first set of water layer multiples and the second set of water layer multiples from the tau-p domain to the t-x domain, and adaptively subtracting the transformed sum of the first set of water layer multiples and the second set of water layer multiples from the gather of seismic data traces in the t-x domain to generate a plurality of primaries in the gather of seismic data traces.
Embodiments of the present invention are also directed to a method for generating a convolutional operator configured to be applied to a gather of seismic data traces. The method includes generating an estimated value of a zero offset two-way travel time in a water layer and an estimated value of a water bottom reflectivity from a water layer model, and computing a convolutional operator using the estimated values of the zero offset two-way travel time and the water bottom reflectivity.